1. Field of the Invention
This invention relates to an electrophotographic lithographic printing plate precursor and a developing method of the same and more particularly, it is concerned with an electrophotographic lithographic printing plate precursor which is suitable for not only the ordinary developing methods but also the developing methods of direct electron injection system and which is capable of obtaining a pinhole-free image and a developing method of the same.
2. Description of the Prior Art
Up to the present time, there has generally been employed an electrophotographic process for the production of a lithographic printing plate precursor comprising subjecting the photoconductive layer of an electrophotographic lithographic printing plate precursor (which will hereinafter be referred to as "master") to uniform static charge, to imagewise exposure and then to liquid development with a liquid toner to obtain a toner image, then fixing this toner image and processing with an oil-desensitizing solution (etching solution) to render hydrophilic a non-image area free from the toner image.
As the base of the above described master, paper rendered electrically conductive has hitherto been used, but this paper base tends to be penetrated with water, resulting in bad influences upon the printability and photographic properties. That is, the paper base is penetrated with the above described etching solution or dampening water during printing and expanded so that the photoconductive layer sometimes separates from the paper base to lower the printing durability, and the moisture content of the paper base is varied with the temperature and humidity conditions in carrying out the above described static charge or imagewise exposure so that the electric conductivity is varied to affect unfavorably the photographic performances.
In order to solve these problems, it has been proposed to coat the one or both surfaces of a paper base with, for example, an epoxy resin or ethylene-acrylic acid copolymer having water resisting property (Japanese Patent Laid-Open Publication Nos. 138904/1975, 105580/1980 and 68753/1984) or to provide with a laminated layer of polyethylene or the like (Japanese Patent Laid-Open Publication Nos. 57994/1983 and 64395/1984).
On the other hand, the above described liquid development has generally been carried out by allowing a developing liquid DL to flow between electrodes 10 and 11 and a master P to pass through the developing liquid DL, as shown in FIG. 3.
In this developing system, however, movement of negative ions in the liquid toner, i.e. developing liquid (negative ions of a polymer added so as to control the charge (+) of a toner in the developing liquid DL, i.e. charge controlling agent) is slow and accordingly, the speed of the negative ions adhering to the surface of a substrate P2 to neutralize positive charges thereof is also slow, so that the adhesion speed of the toner (+) to the surface of a negatively charged photoconductive layer P1 be slow and the master is sometimes conveyed to a next step with toner-nonadhered area left.
In the case of an image which needs adhesion of a toner to a large area such as picture or pattern, in particular, the master is often conveyed to a next step while uniform formation of a so called solid image is not carried out, since when the master P is passing through the developing step, the toner adheres to only a part of the photoconductive layer P1 opposite to a part of the substrate P2 neutralized by adhesion of negative ions and does not adhere to all over the above described large area.
In this developing system, furthermore, the toner (+) gradually adheres to the electrode (-) 11 facing the substrate P2 to lower the developing performance and consequently, it is required to periodically clean the electrode 11.
In order to further solve the above described problems, the inventors have proposed a liquid developing method of direct feed system in which development is carried out by using an elastic conductor 12 such as hardened steel with a diameter of about 0.1 mm as shown in FIG. 2 instead of the above described electrode 11 of the prior art, contacting the conductor 12 with a substrate P2 of a master P, optionally changing a switch and applying a voltage to between the conductor 12 and an electrode 10 from an external power source and directly feeding electrons from the conductor 12 to the substrate P2 (Japanese Patent Application No. 89373/1988).
According to this direct feed system, positive charges on the surface of the substrate P2 are rapidly and surely neutralized with the electrons directly fed from the conductor 12, so that adhesion of toners to the surface of a photoconductive layer P1 can rapidly and surely be carried out and formation of a so-called solid part can uniformly and finely be carried out. In addition, other processings are not required than flowing of a developing liquid DL between the photoconductive layer P1 and electrode 10 and accordingly, the toners (+) do not adhere to the conductor 12.
In the case of the above described direct electron injection system, however, if the substrate P2 has a high electric resistance, electrons cannot directly be fed from the conductor 12 and therefore, it is necessary that the substrate 12 has some electric conductivity.
Of course, in the developing system of the prior art as shown in FIG. 3, if the electric conductivity of the substrate P2 is high to some extent, the neutralization speed of negative ions in the developing liquid DL and positive charges on the surface of the substrate P2 is somewhat increased. Therefore, it is of important significance to decrease the resistance of the substrate P2 in the field of liquid development and this is adapted to not only the above described direct electron injection system but also the prior art developing system. Furthermore, for the purpose of, during imagewise exposure, neutralizing negative charges on an exposed area of the photoconductive layer P1 with positive charges on the surface of the substrate P2 through the interior of the substrate P2, it is important that the substrate P2 has a low electric resistance.
In order to decrease the electric resistance of the substrate P2, it is necessary to render electrically conductive a paper base itself for composing the substrate P2 by adding thereto an electrically conductive material or to decrease the resistance of a coated layer such as of polyethylene provided on one or both surfaces of a paper base to lower the permeability of water as described above by adding an electrically conductive material to the coated layer.
In the case of the coated layer (hereinafter referred to as "undercoated layer") provided on the photoconductive layer side of a paper base, however, an excessively low resistance results in pinholes (spark mark caused by discharge, to which toners do not adhere) and accordingly, it is required that the undercoated layer has at least some surface resistivity. On the other hand, a back layer has no concern for this, and it is not necessary to specify the lower limit of the surface resistivity.